High Efficiency Video Coding (HEVC) is a new coding standard that has been developed in recent years. In the High Efficiency Video Coding (HEVC) system, the fixed-size macroblock of H.264/AVC is replaced by a flexible block, named coding unit (CU). Pixels in the CU share the same coding parameters to improve coding efficiency. A CU may begin with a largest CU (LCU), which is also referred as coded tree unit (CTU) in HEVC. In addition to the concept of coding unit, the concept of prediction unit (PU) is also introduced in HEVC. Once the splitting of CU hierarchical tree is done, each leaf CU is further split into one or more prediction units (PUs) according to prediction type and PU partition. HEVC also supports slice structure, where a picture is partitioned into slices and each slice may use its own coding parameters or configurations.
Along with the High Efficiency Video Coding (HEVC) standard development, the development of extensions of HEVC has also started. The HEVC extensions include range extensions (RExt) which target at non-4:2:0 colour formats, such as 4:2:2 and 4:4:4, and higher bit-depths video such as 12, 14 and 16 bits per sample. One of the likely applications utilizing RExt is screen sharing, over wired- or wireless-connection. Due to specific characteristics of screen contents, coding tools have been developed and demonstrate significant gains in coding efficiency.
Intra Block Copy Prediction
In the current development of screen content coding for High Efficiency Video Coding (HEVC) standard, some tools have been adopted due to their improvements in coding efficiency for screen contents. For Intra blocks, Intra prediction according to the conventional approach is performed using prediction based on reconstructed pixels from neighbouring blocks. Intra prediction may select an Intra Mode from a set of Intra Modes, which include a vertical mode, horizontal mode and various angular prediction modes. For HEVC screen content coding, a new Intra coding mode, named Intra-block copy (IntraBC) has been used. The IntraBC technique that was originally proposed by Budagavi in AHG8: Video coding using Intra motion compensation, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 13th Meeting: Incheon, KR, 18-26 Apr. 2013, Document: JCTVC-M0350 (hereinafter JCTVC-M0350). An example according to JCTVC-M0350 is shown in FIG. 1, where a current coding unit (CU, 110) is coded using Intra MC (motion compensation). The prediction block (120) is located from the current CU and a displacement vector (112). In this example, the search area is limited to the current CTU (coding tree unit), the left CTU and the left-left CTU. The prediction block is obtained from the already reconstructed region. Then, the displacement vector, also named block vector (BV), and residual for the current CU are coded. It is well known that the HEVC adopts CTU and CU block structure as basic units for coding video data. Each picture is divided into CTUs and each CTU is reclusively divided into CUs. During prediction phase, each CU may be divided into multiple blocks, named prediction units (PUs) for performing prediction process. After prediction residue is formed for each CU, the residue associated with each CU is divided into multiple blocks, named transform units (TUs) to apply transforms.
In JCTVC-M0350, the Intra MC is different from the motion compensation used for Inter prediction in at least the following areas:                MVs are restricted to be 1-D for Intra MC (i.e., either horizontal or vertical) while Inter prediction uses 2-D motion estimation.        Binarization is fixed length for Intra MC while Inter prediction uses exponential-Golomb.        Intra MC introduces a new syntax element to signal whether the MV is horizontal or vertical.        
Based on JCTVC-M0350, some modifications are disclosed by Pang, et al. in Non-RCE3: Intra Motion Compensation with 2-D MVs, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 14th Meeting: Vienna, AT, 25 Jul.-2 Aug. 2013, Document: JCTVC-N0256 (hereinafter JCTVC-N0256). Firstly, the Intra MC is extended to support 2-D MVs, so that both MV components can be non-zero at the same time. This provides more flexibility to Intra MC than the original approach, where the MV is restricted to be strictly horizontal or vertical.
In JCTVC-N0256, two BV coding methods are disclosed:                Method 1—Block vector prediction. The left or above BV is selected as the BV predictor and the resulting motion vector difference (BVD) is coded. A flag is used to indicate whether the BVD is zero. When BVD is not zero, exponential-Golomb codes of the 3rd order are used to code the remaining absolute level of the BVD. Another flag is used to code the sign.        Method 2: No block vector prediction. The BV is coded using the exponential-Golomb codes that are used for BVD in HEVC.        
Another difference disclosed in JCTVC-N0256 is that the 2-D IntraBC is further combined with the pipeline friendly approach:                1. No interpolation filters are used.        2. BV search area is restricted. Two cases are disclosed:                    a. Search area is the current CTU and the left CTU or            b. Search area is the current CTU and the rightmost 4 column samples of the left CTU.                        
Among the proposed methods in JCTVC-N0256, the 2-D IntraBC, the removal of interpolation filters, and the search area constraint to the current CTU and the left CTU have been adopted in a new version of draft HEVC RExt standard.
IntraBC in Existing SCC Draft Standard (SCM)
In JCTVC-T0227 (Pang, et al., Non-CE2 Test1: Intra block copy and inter signalling unification, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 20th Meeting: Geneva, CH, 10-18 Feb. 2015, Document: JCTVC-T0227), the IntraBC mode is unified with Inter coding mode. In other words, the current picture is treated as a reference picture and inserted into one or both reference picture lists (i.e., lists L0 and L1). Block vector prediction and coding are treated in the same way as Inter motion vector prediction and coding. This unification simplifies the codec design.
Adaptive Motion Resolution
In JCTVC-S0085 (Li, et al., Adaptive motion vector resolution for screen content, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 19th Meeting: Strasbourg, FR, 17-24 Oct. 2014, Document: JCTVC-S0085), a flag “use_integer_mv_flag” is used for each slice to indicate the resolution of MV in the slice. If this flag is 1, all the decoded motion vectors in this slice are represented using integer-pel resolution; otherwise, the motion vectors are represented using quarter-pel resolution, as specified in the HEVC standard.
Memory Bandwidth Consideration in Current SCC Draft Standard (SCM)
When reconstructing a block, the reference block needs to be used for prediction. If the motion vector points to a fractional position, more pixels surrounding the reference block are also required for interpolation purpose. When retrieving a reference block and its surrounding pixels when necessary from external memory, the data fetch will be conducted in bulk depending on the memory pattern used. For example, if the memory pattern used is 4×4 sample block, the minimum unit for access data will be 4×4 pixels even if only one pixel is needed. Therefore, more data than the direct related pixels will have to be retrieved and sent to on-chip memory/buffer for further processing. These data are all involved in memory bandwidth consumption. In HEVC, the worst case memory bandwidth consumption is in 8×8 bi-prediction mode with both motion vectors having fractional-pel MVs. When IntraBC mode is used, the unfiltered version of current picture needs to be stored in the external memory in addition to the filtered version of current picture for Inter prediction mode. This is considered as an extra cost to HEVC.
In JCTVC-U0078 (Rapaka, et al., CE2: Test 5 on intra block copy constraints on prediction, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 21st Meeting: Warsaw, PL, 19 Jun.-26 Jun. 2015, Document: JCTVC-U0078), some constraints are imposed on the 8×8 bi-prediction mode usage when IntraBC is used. For example, the use of 8×8 bi-prediction mode is disabled when the SPS (sequence Parameter Set) flag for IntraBC is on and slice header flag use_integer_mv_flag (indicating the resolution of MV in the slice) is off.
Decoded Picture Buffer in Current SCC Draft Standard (SCM)
In HEVC, all the reference pictures are stored in a buffer referred as decoded picture buffer (DPB). This is done by each time when decoding a picture, the current decoded picture after the loop filtering operation is put into DPB (referred as filtered version of current decoded picture). For IntraBC, its reference picture is the current decoded picture prior to the loop filter (referred as unfiltered version of current decoded picture), which is an extra picture to be stored in DPB compared to HEVC version 1. In JCTVC-U0181 (Xu, et al., On storage of filtered and unfiltered current decoded pictures, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 21st Meeting: Warsaw, PL, 19 Jun.-26 Jun. 2015, Document: JCTVC-U0181), both the filtered and unfiltered versions of the current decoded picture are put into DPB for picture buffer management. For convenience, the filtered version or unfiltered version of the current decoded picture may be referred as the filtered version or unfiltered version of the current picture by dropping the word “decoded”. The unfiltered version of current picture will be discarded after the completion of decoding the current picture and the storage buffer for this picture will be released.
The case that the two versions of current decoded picture are identical is considered. This occurs when no loop filters (deblocking or SAO) are used for the current picture. In the current HEVC SCC working draft as described in JCTVC-U1005 (Joshi, et al., High Efficiency Video Coding (HEVC) Screen Content Coding: Draft 3, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 21st Meeting: Warsaw, PL, 19 Jun.-26 Jun. 2015, Document: JCTVC-U1005), a variable referred as TwoVersionsOfCurrDecPicFlag is used to identify whether the current picture is used as a reference picture. If so, there may be two different versions of the current picture due to the use of loop filters in the picture.                1) Current decoded picture after the invocation of loop filter is stored in DPB. This picture, upon completion of decoding current picture, is marked as “used for short-term reference”. When the decoding is completed, this picture is stored in DPB for future usage.        2) When TwoVersionsOfCurrDecPicFlag flag is equal to 1, current decoded picture before the invocation of loop filter is stored in DPB, in a separate buffer. This picture is marked as “used for long-term reference” and is used for IntraBC compensation. When the decoding is completed, this picture is removed from DPB.        
According to the HEVC specification, Short-Term Reference Picture Sets are designed to provide reference to pictures in the temporal proximity of the current picture such as within the same structure of pictures. On the other hand, Long-Term Reference Picture Sets can be used to address selected pictures for longer term reference purpose. These long-term reference picture sets are constructed independently from the short-term picture sets. With the changes in decoded picture buffer management when current picture is a reference picture, some constraints need to be imposed to make sure that the function of decoded picture buffer management works properly.